Personally I'd sooner see the Enclave die out than accept wastelanders (I never agreed with the whole killing Vault Dwellers part, particularly in F3 where it makes even less sense) if they comprimse and just become an organisation which accepts anyone they will have lost everything they ever meant to me.
Well, I'm sure they wouldn't do that. But they could setup alliances with groups and not just kill on site. They can even trade say, minor tech, med supplies, to say an organization like talon company to guard their bases.
They aren't Enclave members, but they are serving Enclave will, and is basically cannon fodder for Enclave. Oh, our guards died? Who cares, not Enclave anyway.
They could also work with slavers for same purpose, while if they wanted to recruit, it would be vault people, or exceptionally gifted and useful people, like, the player character....
If I was Enclave, that is what I would do, while preserving the purity of my own organization. And research cloning, baby vats, robots, and uggg maybe even androids.